Recently, with the development of electronic engineering and increased concern for environmental technology, a wide variety of electrochemical devices have been used. In particular, saving energy is most strongly demanded. In the context, it has been increasingly desired to develop techniques that can contribute to energy saving. A lithium-ion secondary battery, which is a representative electricity storage device, has so far been used mainly as a secondary battery for portable devices; however, it has lately been expected to use a lithium-ion secondary battery as batteries for hybrid cars and electric cars.
In conventional lithium-ion secondary batteries operated at around 4 V, a non-aqueous electrolyte having a lithium salt dissolved in a non-aqueous solvent having a carbonate solvent as a main component, has been widely used (see, for example, Patent Document 1). The feature of the electrolyte containing a carbonate solvent resides in good balance between oxidation resistance and reduction resistance and excellent lithium-ion conductivity at a voltage of around 4 V.
It has been desired for lithium-ion secondary batteries to have a further higher energy density. To realize higher energy density, investigation has been made with a view to increasing voltage of the batteries. To increase the voltage of a battery, a positive electrode that can be operated at a high potential must be used. More specifically, various positive-electrode active materials operable particularly at 4.4 V (vsLi/Li+) or more, have been proposed (see, for example, Patent Document 2).